The high cost associated with manufacturing integrated circuits dictates that defective devices be diagnosed as early as possible in the manufacturing process. For this reason, it is highly advantageous to test integrated circuit devices at the wafer level before further processing and packaging. For testing, the wafers are clamped to a wafer chuck and brought into contact with highly sophisticated probes. Once probe contact is made, the circuits are tested for both functionality and power integrity. However, circuits can be tested at power levels in excess of 400 watts, which consequently generates a tremendous amount of heat build-up in the chips which must be removed by the massive wafer chuck.
After wafer test, the wafers are diced into individual devices also known as chips. The good devices are mounted onto substrates to create modules. The modules then go through additional testing that may include burn in and re-test. The test equipment typically includes a high performance heat sink, test socket and tester electronics. The module is installed in the socket, the heat sink is brought into contact with the chip, the chip is tested, the heat sink is removed and the module is removed from the socket and sorted based on functionality. Some or all of these steps are commonly automated.
In some applications, a Liquid Thermal Interface (LTI) is placed between the chip and heat sink in order to improve thermal contact. There are many different types of LTIs that can be used to improve the thermal contact such as, for example, a mixture of water and other additives (i.e., Propylene Glycol (PG)). Water has excellent thermal performance but at high test temperatures may evaporate before the end of test. Also, water and other substances such as, for example, PG, can corrode the C4 connects. The corrosion or evaporation often happens only on small areas of the device and only on a small number of the total devices tested in a high volume manufacturing environment.
Alternate interface materials can have other disadvantages. Helium is clean and non-corrosive but thermal performance is insufficient. PAO (Poly Alpha Olefin) oil and various types of thermal grease have thermal performance almost as good as water but require cleaning with a solvent after test. Thermal pads have insufficient thermal performance and often leave a residue. Fluorinated fluids may be non-corrosive and clean but have poor thermal performance due to their low thermal conductivity. Liquid metals and soft metals can have very good thermal performance but can oxidize (degrade) over time and repeated reuse, they can damage the heat sinks and can have unreliable thermal contact to the chip. Greases, phase change materials and adhesives can make it difficult to separate the heat sink from the chip after test and leave a residue.
If faults stemming from the improper use of LTI are found after production has started it may be necessary to recall product and develop a new LTI resulting in significant production delays. Thus there is a need for improved methods of evaluating LTI fluids to ensure they will provide reliable service. Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.